Terminology: Irradiance and Irradiation

Irradiation is the energy received per area. It is expressed in J m-2 (joule per square meter). The International Solar Energy Society recommends the symbol H. Other units are J cm-2 (joule per square centimeter) and Wh m-2 (watt-hour per square meter) though the latter should not be used as it does not belong to the standard unit system (S.I., International System). See conversion factors.

Irradiance is defined as a power received per area. It is expressed in W m-2 (watt per square meter). The S.I. (International System) recommends the symbol E.

Irradiance and irradiation values stand for area densities, i.e. no subscripts are used to indicate the surface area. All symbols refer to horizontal planes; for a tilted plane, the slope β (beta) and the plane azimuth α (alpha) are added in brackets. Subscript 0 stands for extraterrestrial or astronomical. Subscript h stands for hourly and subcript d for daily.

The conversion from irradiation (energy) into irradiance (power) is performed by dividing irradiation by the duration of the measurement. Reciprocally, irradiance is converted into irradiation by multiplying by a time duration.

To convert 10-days sums of irradiation, or monthly sums or yearly sums or similar quantities, into irradiance, convert them first to averaged values by dividing them by the duration of the observation (e.g., 10 days, 28 or 30 or 31 days, 365 or 366 days). Then use the table above.

Units

International system of units

The International System of Units, abbreviated SI (from the French Système International), is the internationally-agreed system of measurement.

Angles: Plane angles are expressed in radian (rad), or degrees, or sexagesimal degrees (degrees, minutes, seconds).Energy: Energy, work, quantity of heat are expressed in Joule (J). It can be expressed by unit of surface: J/m2, J/cm2. Irradiation is the energy received per area. Other units are Wh (Watt hour), calorie, therm, Langley.Power: Power, radiant flux are expressed in Watt (W). Irradiance is defined as a power received per area; it is expressed in W m-2 (watt per square meter).Pressure: Pressure is expressed in Pascal (Pa). In meteorology, the unit hPa (hecto-Pascal) is commonly used; it is 100 times a Pascal. It replaces the millibar unit and they are equivalent. Another unit is the "atmosphere".